Liquid developer composition for electrostatic images



United States Patent 01 Efice U.S. Cl. 252-621 5 Claims ABSTRACT OF THE DISCLOSURE A method and composition for development of a latent electrostatic image formed on a stratum having an organic binder component comprising development with a liquid developer in which the liquid portion is formulated with a major portion of a low boiling liquid which is a nonsolvent for the organic binder and from 0.5 to 5 percent by weight of a higher boiling liquid in which the organic binder is soluble, and in which the developing liquid has the desired resistivity of ohm-cm. or more whereby the lower boiling liquid evaporates off more rapidly than the higher boiling liquid which thus increases in concentration towards the end of the drying cycle to the level whereby it becomes efliective to reduce the organic binder to a state for fixing the tinctorial components.

This invention relates to electrostatic printing and more particularly to an improved liquid developer composition for converting a latent electrostatic image to a visible image and for fixing the visible image to produce permanent and smudge-free copy.

Processes for forming a latent electrostatic image, existing as an electrostatic charge pattern in a layer of material having high resistance, and for subsequently converting the latent electrostatic image into a visual pattern, are well known.

In one such process, an optical image is produced direct- 1y on a film or coating of a photoconductive pigment, such as zinc oxide dispersed in an insulating matrix, such as a synthetic resin, on the surface of a flexable carrier, such as a sheet of paper, metal or other electrically conductive backing. In this process, the photoelectrostatic coating is given an overall electrostatic charge while being protected from light and thereafter it is exposed to a light image of the subject to be reproduced. The electrostatic charge on the coating is dissipated in the areas struck by light and retained in the unexposed areas thereby to provide an electrostatic reproduction of the optical image. This latent electrostatic image is then converted to a visible image by a developing composition containing toner particles which are attracted to the latent electrostatic image in a positive toning process, or to the background area in a negative toning process.

In another process referred to as the Videograph process, described in U.S. Pats. No. 2,996,573 and No. 3,075,859, a latent electrostatic image is inscribed by elements extending through a cathode ray tube directly onto the dielectric coating on the surface of a base sheet of paper or other highly electrically conductive material The latent electrostatic image is retained on the dielectric coating and later developed into a visible image by the application of a suitable developing composition containing toner particles which are attracted to the latent electrostatic image or the background, as described above.

Liquid developer compositions for use with electrostatic images comprise a dispersion of a pigment or toner particles in a volatile liquid having a high dielectric strength and a high volume resistivity. The dispersed particles may carry either a positive charge or a negative electrical charge, depending on their chemical composition 3,535,244 Patented Get. 20, 1970 for either negative toning or positive toning respectively. The non-conductivity and the high dielectric strength of the volatile liquid in the liquid developing composition preserves the electrostatic image and permits the deposition of the dispersed toner particles to form a visible image. Liquid toners are described in the U.S. Metcalfe Pat. No. 2,907,674, the U.S. Straughan Pat. No. 2,899,- 335, the U.S. Mayer et al. Pat. No. 2,890,911, the U.S. York Pat. No. 3,135,095 and the U.S. Dirks Pat. No. 3,155,546, and many others.

Although liquid developing compositions represent an improvement over the use of earlier dry powder developers used for conversion of a latent electrostatic image to a visible image, none of the liquid developers are entirely satisfactory from the standpoint of producing a permanent visible image which is free of smear or blurring especially when rubbed. This stems from the inability of the toner particles to become permanently fixed on the surface of the copy sheet. Attempts have been made to fix the image by subsequent processing as by heating to fuse the toner particles, but such additional processing steps are insuflicient and often incomplete.

Further attempts have been made to formulate the liquid developing composition with a resinous or polymeric component dissolved therein as a binder or with a liquid system containing a solvent which partially dissolves or softens a resinous component contained in the suspension or forming a part of the pigment particles for the purpose of bonding the particles onto the copy sheet. The presence of such dissolved or partially dissolved organic binder components brings about agglomeration of the toner particles with resulting separation in the developing composition and non-uniformity in the deposition of the toner particles for development of the visible image.

Liquid developers containing dissolved resinous or hinder components dry on the applicator rolls to cause staining of the original copies, cobwebbing in operation of the applicator rolls, and sticking of the rolls. When the developing composition contains solvent for the resinous component in suificient amount to retain the resin in solution in the developing composition, the composition is capable of attack of the insulating or dielectric layer to provide excessive background color.

It is an object of this invention to produce a developing composition for latent electrostatic images wherein the deposited toner particles become permanently fixed to the copy sheet to produce good readable copy; wherein additional processing steps are not required to fix the toner particles deposited during development of the image; in which the developing solution is stable and relatively free from agglomeration or separation of the toner particles; in which the image becomes fixed during normal drying of the copies; and in which the developing composition is otherwise the same as compositions heretofore employed from the standpoint of characteristics and use so as to enable application in the conventional manner without change in equipment, procedure, or processing steps The invention results from the approach to the problem from an entirely different direction than that heretofore employed. Instead of the incorporation of ingredients into the developing composition, which operate to fix the toner particles onto the surface of the copy sheets, as by the formulation of the developing composition to contain a resinous binder component that is activated by a liquid ingredient in the composition or by subsequent response to heat or solvents, utilization is made of the resinous or polymeric binder making up the dielectric coating on the Videograph paper or present as a binder for the zinc oxide particles in the photoconductive layer to render the resinous or polymeric binder adhesive during the final stages of drying of the developing composition whereby the deposited toner particles are retained by the underlying coating to fix the visible image onto the copy sheet. This enables the developing composition to be formulated of materials which will not cause agglomeration of the toner particles thereby to permit formulation of a stable and uniform developing composition and it also enables the formulation of a developing composition which is free from attack on the coating thereby to minimize the development of background color.

The concepts of this invention reside in the formulation of a developing composition in which the toner or pigment particles are suspended in a liquid carrier in which the organc binder component of the dielectric coating or of the photoconductive coating is completely insoluble and otherwise unaffected by the liquid carrier, but in which the liquid carrier is formulated to contain an ingredient which is capable of rendering the resinous binder adhesive by softening or partial solution but in which said ingredient is present in such small concentration in the original liquid carrier as to have no eifect on the binder component of the coating, but in which the said liquid component has an evaporation rate considerably less than that of the remainder of the components of the liquid carrier whereby the proportion of said component increases in the liquid carrier remaining on the coating during drying to the end that said component becomes effective during the final stages of drying to re duce the binder, in at least the surface portions of the coating, to an adhesive or tacky stage. Thus the deposited toner becomes adhesively secured to the coated surface by the binder component originally present in the dielectric or photoconductive coating permanently to fix the toner particles in the copy.

The normal requirements for the liquid carrier are still retained in that the liquid carrier system must still be inert and have a high volume resistivity in excess of 10 ohms-cm. so as to avoid dissipation of the charge in the latent electrostatic image. For this purpose, it is desirable to formulate the liquid carrier in the developing composition with the major proportion of the liquid carrier formed of a non-solvent diluent component or components which have the desired volume resistivity and which are completely inert to the resinous or polymeric component in the coating or to any of the resinous materials which might be present in the development composition, either as a component of the composition or as a part of the toner particles. In order to achieve the desired effect, it is important to make use of a fixing component which is compatible with the non-solvent materials but which has an evaporation rate, as evidenced by a vapor pressure or boiling point, which is considerably higher than the non-solvent material, otherwise insuflicient of the fixing component will remain and the ratio during the final stages of drying will be insufficient to reduce the resinous component in the coating to the desired tacky or adhesive stage. For this purpose, it has been found best to make use of a fixing component in the liquid carrier in an amount corresponding to 0.5 to percent by weight of the liquid carrier and preferably in an amount within the range of 1 to 3 percent by weight. When the amount of fixing component exceeds 5 percent by weight of the liquid carrier, the system is not completely inert to the binder materials which may be contained in the developing composition or toner particles whereby instability occurs due to agglomeration or the like and/or the liquid carrier will not be inert to the binder component of the coating whereby the developing composition will be retained by the non-imaged portions of the coating to develop an undesirable background color. When less than 0.5 percent by weight of the fixing material is present in the liquid carrier, the amount remaining after most of the diluent has been evaporated off will be insufficient to 4 effect the desired reduction of the binder component to an adhesive or tacky stage for fixing the toner particles onto the copy sheet.

When the fixing component is employed within the range described, its presence in the liquid carrier will be so insignificant as to have little, if any, effect on the inertness of the developing composition to the polymeric ma terials which might be present in the developing composition or as a binder in the dielectric or photoconductive coating of the copy sheet. Thus the developing composition can be applied for exposure to the latent electrostatic image to enable toner particles to travel for retention by the electrostatic image while the remainder flows cleanly from the non-imaged portions of the plate to produce clean copy of good quality. The developing composition remains stable without solution of the binder from the coating or in the composition or toner particles whereby uniformity in composition is retained throughout the reproduction process.

When, as in normal practice, the binder component in the photoconductive layer or in the dielectric film or coating is formulated of such materials as polyvinyl acetate, polyvinyl acetate copolymers containing carboxyl groups, polyvinyl cholride-polyvinyl acetate copolymer, polystyrene, styrenebutadiene copolymer, alkyd and modified alkyd resins, synthetic rubbers such as Pliolite or other resinous systems insoluble in aliphatic solvents and soluble in aromatic solvents, the non-solvent component of the liquid carrier is selected of low boiling aliphatic hydrocarbons such as isopentane, octane, cyclohexane, aliphatic petroleum solvents and paraflinic solvents, or preferably a solvent having a KB. value of not more than 50 and having a boiling point within the range of 200- 400 F. and preferably within the range of 250250 F. For purposes of raising the flash point to safe levels above F. and also to increase the volatility or the evaporation rate of the solvent system, it is desirable to include, as a part of the non-solvent component, such halogenated hydrocarbons as the chlorinated-fluorinated ethanes and methanes, marketed by the E. I du Pont de Nemours & Co. under the trade name Freon such as Freon TF (trichlorotrifluoroethane, having a boiling point of F.), Freon MF (trichloromonofluoromethane, having a boiling point of 75 F.), etc.

As the fixing component of the liquid carrier, use can be made of high boiling aromatic solvents having a K.B. value in excess of 80. While the boiling point is not critical, it is preferred to make use of a solvent component having an initial boiling point in excess of about 350 F. Such components have the desirable high electrical resistivity, low dielectric constant, and miscibility with the aliphatic non-solvent component or components and the desired tackifying characteristics for the binder component normally used in the dielectric insulating 0r photoconductive coating. Representative of suitable aromatic solvents are such materials as Panasol AN-l, marketed by American Oil Company (boiling point range 400-494 F., K.B. value of 108 and flash point temperature of F. calculated by the Cleveland open cup method); Espesol 4, marketed by Signal Oil Company (90% aromatic, having a K.B. value over 90, a boiling point range of 446-555 F., and an open cup flash of 200 F.); Comsolv #7, marketed by Commerce Petroleum Company (having a boiling point range of 441-665 F., a K.B. value of 157 and a flash point of 245 F.); Comsolv #8 aromatic solvent, having a boilmg point range of 367526 F., a K.B. value of 96 and a closed cup flash point of 167 F.

Other materials which can be employed as the fixing component of the liquid developer comprise high boiling ethers and esters such as butyl Carbitol, Carbitol, butyl Cellosolve, butyl Carbitol acetate, butyl Cellosolve acetate, but use of the above is limited to developing compositions employed with coatings containing binders which are somewhat soluble or plasticized by such glycol ethers and esters, such, for example, as films or coatings formed of polyvinyl acetate. While the electrical resistivity of such glycol ethers and esters is sometimes less than 10 ohm.- cm., the fixing component is present in such small amounts in the liquid carrier as to have little detrimental effect.

The toner particles of the developing composition can be selected of a wide variety of solid particles. Suitable organic or inorganic materials are described in the Carlson Pat. No. 2,297,691, including talcum powder, aluminum bronze, carbon dust and the like, with the principal requirement being that the particles be electrically attractable. It is preferred to make use of a powdered dyestuif, such as nigrosines, or a carbonaceous material, such as carbon black, lamp black, channel black or the like. The toner particles may be in the form of pigment particles formulated of a suitable dyestulf or carbon black embodied in a resinous carrier, such as described in US. Pats. No. 2,907,674, No. 2,891,911, No. 3,135,095, and No. 2,899,355. The amount of toner particles in the developing composition is extremely low, usually within the range of .05 to 2 percent by Weight.

Having described the basic concepts of the invention, illustration will now be made of the formulation and use of a developing composition representative of the practice of this invention. The following composition in Example 1 finds excellent use with an insulating stratum in which the' resinous binder component is a polyvinyl acetate copolymer containing carboxyl groups:

EXAMPLE 1 Percent by Weight Toner concentrate Aliphatic solvent (petroleum fraction, having a boiling point range of 250350 F.) 43 Trichlorotrifluoroethane (Freon TF, boiling point 115 F.) 25 Tricholromonofluoromethane (Freon MF, boiling point 75 F.) 25 Aromatic solvent (boiling point range 400-494 The concentrate is merely mixed with the remainder of the diluents making up the liquid carrier. Preparation of the concentrate is represented by the following:

EXAMPLE 2 Percent by weight Liquid oil black (9096-Calco Chemical Com- P y) Carbon black Alkyd Blue 746 (alkali blue toner flushed with an alkyd resin) 7.5 Shell Sol 71 62.5

In the foregoing example, the Calco Chemical Company liquid oil black is a mixture of basic nigrosine dye, 2530 parts by weight, and oleic acid, 75-70 parts by weight. The Shell Sol 71 is a 100% parafiinic fraction having a boiling point within the rang of 346-400 F., and a K.B. value of 26.

EXAMPLE 3 Percent by weight Liquid oil black 20 Channel black 5 Shell Sol 71 75 In use, the copy sheet containing the latent electrostatic image is Wet with the liquid developing composition, either by immersion of the sheet in a bath of the developing composition or by flow-coating the composition over the imaged surface, or by application of the liquid developing composition onto the imaged surface by a roller coater. The copy sheet wet with the developing composition is advanced through a squeeze roll to remove excess liquid. The toner particles are attracted to the latent electrostatic image for visual development of the image.

After development, the copy is allowed to dry by evaporation of the liquid, with or without the existence of heat. The lower boiling non-solvent component evaporates oif at a much greater rate than the fixing component. As a result, during the final stages of drying, the ratio of the fixing component to the non-solvent component is increased to the extent that the remaining solvent system becomes effective to reduce the binder in the coating to a tackified or adhesive stage either by solution, partial solution, or plasticization of the binder component in at least the surface portion of the coating. Thus the deposited toner particles become bonded to the underlying coating to become permanently fixed on the copy sheet upon complete drying of the sheet.

The concept of this invention enables return to the more stable developing compositions which are free of any dissolved resinous binder component intentionally incorporated for use as a fixing agent to secure the toner particles upon drying. Such developing compositions which are free of dissolved binder are free of the undesirable characteristics of agglomeration of toner particles, coating of the rollers, webbing of the rollers, sticking of the rollers and development of background color, and such developing compositions exhibit greater stability and greater uniformity in storage and in use.

Developing compositions containing resinous binder as a part of or in admixture with the toner particles may be employed in the practice of this invention but under the conditions that little, if any, of the resinous component is dissolved or softened in th e liquid carrier composition, at least until most of the non-solvent component has been evaporated off to leave the fixing component in high concentration suificient to reduce the resinous component to an adhesive or tacky stage in addition to tackifying the binder component in the coating for fixing the toner particles.

It will be apparent from the foregoing that the concepts of this invention for fixing the toner particles in the visible image offer a number of very important advantages over systems which have previously been employed, namely (1) development of electrostatic images with good intensity and permanency without the benefit of dissolved resins for fixing or for charge-directing purposes;

(2) the formulation of a liquid developer characterized by good stability;

(3) high speed development by reason of the absence of dissolved resinous or binder components in the liquid developing composition;

(4) clean intermittent development by reason of the absence of film deposits on the machine rolls;

(5) permanent image by reason of the toner particles being locked into the insulating surface;

(6) absence of attack of the insulating coating on the surface of the copy sheet during development;

(7) fixing of the image in response to the normal drying operation; and

(8) cobwebbing is eliminated by reason of the absence of a dissolved resinous component in the liquid developing composition.

It will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims:

I claim:

1. A composition for visual development of latent electrostatic images formed of a stratum having an organic binder component selected from the group consisting of polyvinyl acetate, polyvinyl acetate copolymers containing carboxyl groups, polyvinyl chloride-polyvinyl acetate copolymer, polystyrene, styrene-butadiene copolymer, and alkyd resins consisting essentially of toner particles suspended in a liquid carrier formulated of a major proportion of a low boiling liquid having a K.B. value of less than 50 which is a non-solvent for the organic binder and having a boiling point within the range of 200-400 F. and selected from the group consisting of an aliphatic solvent and a mixed aliphatic-chlorinated fluorinated ethane solvent and a chlorinated-fluorinated ethane, and

from 0.5 to 5 percent by weight of an aromatic solvent having 3. KB. value in excess of 80 and a boiling point above 350 F. in which the organic binder component is soluble, the organic binder being insoluble in the liquid carrier formulated of the solvent and non-solvent components.

2. The method for development of latent electrostatic images on a stratum containing an organic resinous binder comprising contacting the surface of the stratum containing the latent electrostatic image with a composition containing toner particles suspended in a liquid carrier having a volume resistivity in excess of 10 ohms/ cm. and which is formulated of a low boiling liquid component which is a non-solvent for the organic resinous binder and another higher boiling liquid component present in an amount within the range of 0.5 to 5 percent by weight which is a sol vent for the organic resinous binder, the combination of liquids having a carrier in which the organic resinous binder is insoluble, evaporating oc liquid carrier and, in the process of evaporation, the lower boiling liquid evaporates off at a rate greater than the higher boiling liquid to the end that the relative proportion of the higher boiling liquid increases near the end of the evaporation whereby the organic resinous binder is reduced to an adhesive stage to set the image.

3. The method as claimed in claim 2 in which the nonsolvent component is an organic aliphatic solvent nd in which the higher boiling solvent component is an aromatic solvent.

4. The method as claimed in claim 2 in which the nonsolvent component has a boiling point within the range of 250-350 F. and in which the solvent component has a boiling point in excess of 350 F.

5. The method as claimed in claim 2 in which the evaporation is carried out by heating to elevated temperature.

References Cited UNITED STATES PATENTS 3,192,043 6/1965 Metcalfe et al 961 3,296,140 1/1967 Zabiok 25262.l 3,058,914 10/1962 Metcalfe et al. u--- 25262.1 3,105,821 10/1963 Johnson 25262.1

JOHN DAVID WELSH, Primary Examiner US. Cl. X.R. 

